B cell antigen receptor (BCR) signaling controls the development, selection and function of B lymphocytes. Work in our lab has determined that the lysophospholipid, lysophosphatidic acid (LPA), signals to the LPA5 G-protein coupled receptor expressed by B lineage cells to suppress BCR signaling and subsequent antibody response. LPA binds and signals to LPA receptors with low nanomolar affinity and as a major lysophospholipid is present in blood at high nanomolar to low micromolar concentrations. However, despite that lymphocytes express several LPA receptors; relatively little is understood about how LPA influences humoral immunity. A long-term goal of our research has been to understand how BCR-derived signals intersect with those signals transmitted via GPCRs such as chemoattractant receptors and the goal of this application is to define how LPA signaling through LPA receptors on B lineage cells regulates the development and function of B lymphocytes. LPA has also been characterized as an inflammatory lipid and whose levels are considerably elevated in a number of chronic inflammatory disorders such as cancer, autoimmunity and viral infections. We show that at these heightened LPA levels BCR signaling is further inhibited. Thus, we also investigate how pathophysiological levels of LPA alter B cell tolerance induction in the bone marrow and antibody responses by marginal zone and follicular B cell populations. To address these issues, we propose in vitro and in vivo experiments that rely on well-characterized mouse models of B cell tolerance and antibody response to elucidate the role(s) of LPA receptors on B lymphocytes during their development in the bone marrow and their function as mature B cells in the periphery. These experiments are outlined in the following Specific Aims: Aim 1: Characterize how LPA regulates immature B cell development and tolerance induction. 1A. Establish if LPA receptors guide immature B cell localization in the bone marrow during These experiments ask how LPA receptor signaling influence B lymphopoeisis and if pathological LPA levels alters central B cell tolerance. Aim 2: Characterize the molecular and cellular mechanisms that lead to LPA suppression of B cell antibody responses. Here we define the extent to which LPA regulates B cell responses and the signaling pathways used by LPA receptors to inhibit BCR signaling. In particular, we determine if antigen-specific B cell responses are suppressed by all antigens or only antigens with certain (weak) affinity. Aim 3: Characterize how local autotaxin expression influences LPA regulation of B cell responses and if inflammatory and autoimmune settings alter the regulation of its expression. These experiments are designed to define the relative contributions of locally-restricted LPA production versus global systemic LPA levels in regulating the B cell antibody response and how LPA production may be altered in inflammatory and autoimmune settings. Significance. The sphingosine-1-phosphate (S1P) lysophospholipid has emerged as a critical regulator of lymphocyte development, trafficking and localization. However, lymphocytes and most other cells of the immune system also express G-protein coupled receptors that recognize another major lysophospholipid, lysophosphatidic acid (LPA). In contrast to S1P, that serves an important role under homeostatic conditions, LPA has features of an inflammatory lipid and has been associated with a chronic inflammatory disorders. Notably, how LPA regulates immune function and, specifically, humoral immunity is largely unexplored. The successful completion of these experiments is expected to illustrate how LPA functions to regulate the development, selection and antibody response by B lymphocytes. Furthermore, as LPA-LPA receptor signaling has been associated with a number of different types of cancer and has received considerable attention for possible therapeutic intervention, these findings will also be important to inform on how such strategies might alter adaptive immunity.